Ancient events and climate adaptive capacity shaped distinct chloroplast genetic structure in the oak lineages

BackgroundUnderstanding the origin of genetic variation is the key to predict how species will respond to future climate change. The genusQuercusis a species-rich and ecologically diverse woody genus that dominates a wide range of forests and woodland communities of the Northern Hemisphere.Quercusthus offers a unique opportunity to investigate how adaptation to environmental changes has shaped the spatial genetic structure of closely related lineages. Furthermore,Quercusprovides a deep insight into how tree species will respond to future climate change. This study investigated whether closely relatedQuercuslineages have similar spatial genetic structures and moreover, what roles have their geographic distribution, ecological tolerance, and historical environmental changes played in the similar or distinct genetic structures.ResultsDespite their close relationships, the three main oak lineages (QuercussectionsCyclobalanopsis,Ilex, andQuercus) have different spatial genetic patterns and occupy different climatic niches. The lowest level and most homogeneous pattern of genetic diversity was found in sectionCyclobalanopsis, which is restricted to warm and humid climates. The highest genetic diversity and strongest geographic genetic structure were found in sectionIlex, which is due to their long-term isolation and strong local adaptation.The widespread sectionQuercusis distributed across the most heterogeneous range of environments; however, it exhibited moderate haplotype diversity. This is likely due to regional extinction during Quaternary climatic fluctuation in Europe and North America.ConclusionsGenetic variations of sectionsIlexandQuercuswere significantly predicted by geographic and climate variations, while those of sectionCyclobalanopsiswere poorly predictable by geographic or climatic diversity.Apart from the different historical environmental changes experienced by different sections, variation of their ecological or climatic tolerances and physiological traits induced varying responses to similar environment changes, resulting in distinct spatial genetic patterns.